Most cancers arise by an evolutionary process as genetic and epigenetic changes accumulate in somatic cells allowing them to escape the proliferative restrains that control cell growth. In recent years it has become evident that one critical barrier to cancer progression is a proliferative arrest termed cellular senescence. Our studies have demonstrated that the reasons for the inactive nature of certain human cancer precursor lesions, such as melanocytic nevi, ductal hyperplasias of the breast, and colonic adenomas is because cells within these lesions had undergone telomere dysfunction-induced senescence (TDIS). Yet, given that cells within these lesions occasionally continue to proliferate and thereby allow these neoplasms to progress to more advanced cancer stages, it is likely that cells can escape TDIS following a long period of inactivity. Indeed, our preliminary data demonstrate that, depending on the signaling pathways activated in senescent cells, TDIS is not always stable and cells can escape this proliferative arrest following a prolonged period in senescence. Surprisingly, senescent cells acquire a gene expression signature that in part resembles that of stem cells, suggesting that senescent cells undergo epigenetic changes that provide cells with stem cell-like characteristics. In order to better understand the molecular changes that promote escape from senescence and to predict which lesions remain inactive virtually indefinitely and which have the potential to progress to more advanced cancer stages, we propose to identify the stages during breast, colon, and (melanocytic) skin cancer development in which the telomere-initiated senescence responses are inactivated. We will use this knowledge to improve diagnostic and prognostic tools that evaluate cancer stage and potential for cancer progression, develop novel biomarkers for cancer stage, and facilitate decision making for patient treatment. Additionally, we will compare transcriptomes, epigenomes, and exomes from normal, senescent, and senescence-escaped cells, both from cell cultures and isolated from human tissue, in order to characterize the earliest changes that result in inactivation of the tumor suppressing functions of TDIS. A thorough understanding of the changes that promote senescence escape will allow us to facilitate development of novel anti cancer strategies and/or improving existing ones. Finally, we will use cell culture and mouse model systems to characterize escape from telomere-initiated cellular senescence in greater detail and in a physiologically relevant setting. These model systems will allow us to not only differentiate changes that are causative from those that are a consequence of senescence escape, but they will also provide a platform for testing drugs and therapies that target malignant cancer growth at its earliest stages.